It is advantageous to keep the rotational speed of of an internal combustion engine crank shaft at a desired value during an idling operation. The desired idling speed provides for the stable operation of the engine, eliminates unnecessary fuel consumption and reduces harmful components contained in the exhaust gases. During an idling operation, the amount of fuel, i.e. the fuel flow ratio, is determined by an idle adjust screw disposed at an idle port of a carburetor slow circuit. The idle adjust screw is so adjusted that a predetermined amount of fuel is fed into the intake manifold so that the engine rotates at a predetermined speed which is most desirable. However, the engine speed in a idling operation is apt to vary due to various reasons, such as variations of atmospheric pressure, ambient temperature, and engine temperature. For instance, the amount of fuel and/or air induced into the intake manifold of the engine are apt to vary due to deterioration of various members such as the throttle valve and the cylinders of the engine. Further, the ignition timing influences the engine speed.
In a conventional carburetor system, the idle adjust screw has to be adjusted each time the engine speed in an idling operation differs from the preset value. It is troublesome to manually adjust the screw which is disposed in the carburetor.